1. Field of the Invention
The invention relates to an apparatus for testing integrated circuits (IC), and more particularly to an apparatus for testing servo motor control integrated circuits (IC).
2. Description of the Related Art
A servo motor control integrated circuit (IC) determines output signals and a pulse width of each output signal according to an input signal with fixed period and pulse width modulation (PWM). A dead band (DB) region occurs when all output signals are deasserted in the servo motor control integrated circuit.
FIG. 1 is an exemplary pin diagram of a conventional servo motor control IC 100. A first pin is an input terminal for receiving an input PWM signal INPWM with fixed period. Second and third pins are output terminals for respectively generating a first output signal OUT1 and a second output signal OUT2 according to the input signal INPWM. The first output signal OUT1 and second output signal OUT2 are not asserted at the same time. A fourth pin is the output terminal for outputting an analog error signal EA.
FIG. 2 is a waveform diagram of each signal of the conventional servo motor control IC. In FIG. 2, label II represents a dead band state in which the IC operates in the dead band region and the first output signal OUT1 and second output signal OUT2 are deasserted. Label III represents a second state in which only the second output signal OUT2 is asserted, and label I represents a first state in which only the first output signal OUT1 is asserted. For example, the IC is operated in the dead band region if the period of input signal INPWM is 20 ms and the pulse width of input signal INPWM is 1.5 ms.
As shown in FIG. 2, in the dead band state the IC operates in the dead band region when the pulse width of input signal INPWM is 1.5 ms, and the first output signal OUT1 and second output signal OUT2 are deasserted. In the first state the first output signal OUT1 is only asserted if the pulse width of input signal INPWM is less than 1.5 ms. In the second state the second output signal OUT2 is only asserted if the pulse width of input signal INPWM is more than 1.5 ms.
The ability to detect the window of a dead band region is critical for servo motor control ICs. Each servo motor control IC, for example, must have three states as described in FIG. 2. FIG. 3 is a conventional apparatus for testing servo motor control IC 310. In FIG. 3, an output terminal of tester 320 is coupled to a first pin of IC 310, and second and third pins of IC 310 are coupled to the input terminals of tester 320 respectively. Tester 320 transmits the input signal INPWM to IC 310, and receives the first output signal OUT1 and the second output signal OUT2 from 310. Tester 320 can then gradually modulate the pulse width of input signal INPWM; thus, IC 310 can operate separately in the first, second, and dead band states. The dead band state contains the entire dead band region.
FIG. 4 is an exemplary waveform diagram of testing signals of conventional servo motor control IC 310. First, the pulse width of input signal INPWM output from tester 320 is 1401 μs, as shown in FIG. 4. Tester 320 subsequently receives the first output signal OUT1 from IC 310. Referring back to FIG. 2, only the first output signal OUT1 is asserted within the first state, i.e. the pulse width of input signal INPWM is shorter than the pulse width of dead band region. Thus, tester 320 gradually increases the pulse width of input signal INPWM by one micro-second at a time. IC 310 can operate in the dead band state until the pulse width of input signal INPWM is increased to 1501 μs, and the first output signal OUT1 and second output signal OUT2 are deasserted. After that, tester 320 continuously increases the pulse width of input signal INPWM gradually until the second output signal OUT2 is asserted. In FIG. 4, the dead band region is from 1501 to 1508 μs, and the dead band region interval is 8 μs.
The region and interval of dead band of each servo motor control IC, due to process variability, are not the same. For example, the dead band region of servo motor control IC ranges between 1530 μs and 1534 μs or between 1487 μs and 1495 μs. For this reason, the pulse width of input signal INPWM must contain the total dead band region of the IC. For example, the pulse width of input signal INPWM from tester 320 ranges between 1401 μs and 1600 μs, and is adequate to contain the total dead band region of the IC, i.e. 200 fixed cycles are required to test one IC. Testing time, however, increases with an increased pulse width modulation region. Conversely, decreasing the pulse width modulation range may not detect dead band regions and thus reduce yield.